The present invention relates to a motor of which rotation speed and rotary phase can be adjusted.
Motors have been used for many applications in our daily life. While characteristics of a motor depend on the particular application of the motor, stability of a rotation speed and a rotary phase typically affects properties of the motor. For example, motors used to drive a magnetic head of video tape recorders or rotary digital audio tape recorders should be capable of being rotated at a constant speed. This is because an inferior rotary characteristic of the motor causes a reproduced signal to be changed such that reproduced images or sounds are caused to fluctuate.
The motor of the type described above typically comprises a shaft rotatably supported by a supporting member, first and second rotor members, and first and second stator members. The first and second rotor members are secured to the shaft and are opposed to the first and second stator members, respectively. The first rotor member has a driving magnet assembly attached thereto. The driving magnet assembly comprises an even number of driving magnetized elements. The driving magnetized elements are aligned on a perimeter or circumference of the first rotor member about the shaft such that one driving magnetized element has a pole opposite the pole of the neighboring ones. In other words, the driving magnet assembly has alternating poles. The first stator member has an armature which comprises armature coils opposed to the driving magnetized elements such that a predetermined gap is formed between the armature coils and the driving magnet assembly. An alternating current across the armature coils produces induction field, which generates an attractive (opposing) force between the driving magnetized elements and the armature coils. This force causes the first rotor member to rotate.
The second rotor member has a sensed magnet assembly attached thereto. The sensed magnet assembly comprises an even number of sensed magnetized elements. The sensed magnetized elements are aligned on a perimeter or circumference of the second rotor member about the shaft such that one sensed magnetized element has a pole opposite the pole of the neighboring ones. The second stator member has a magnetic field sensor opposed to the sensed magnet assembly such that a predetermined gap is formed between the magnetic field sensor and the sensed magnet assembly. The magnetic field sensor comprises a plurality of sensing elements for sensing the alternating magnetic field with rotation of the second rotor member. The magnetic field causes an induced electromotive force to be generated. This induced electromotive force is used to produce a sensor signal corresponding to the rotation of the rotor members and then determine the rotation speed and/or the rotary phase of the motor.
As apparent from the above, the conventional motor has separate magnet assemblies for driving as well as for sensing. This increases the number of components or parts of the motor assembled through complicated steps. Such motor can be produced only at a relatively large production cost. In addition, these two magnet assemblies limit available reduction in size of the motor itself. Further, the number of the sensing elements is relatively small due to economical considerations. It is thus difficult to distinguish the sensor signal from a noise when the rotor members rotate at a low speed. More specifically, the induced electromotive force has a waveform of relatively small amplitude due to the low number of the sensing elements especially in rotation at a low speed. This makes it hard to distinguish the sensor signal from noise.